The invention relates generally to solid oxide fuel cell systems and particularly to a compact thermal management and fuel reformation system for solid oxide fuel cell systems suitable for use in numerous applications including transportation applications.
Fuel cells are electrochemical devices that convert chemical potential energy into usable electricity and heat without combustion as an intermediate step. Fuel cells are similar to batteries in that both produce a DC current by using an electrochemical process. Two electrodes, an anode and a cathode, are separated by an electrolyte. Like batteries, fuel cells are combined into groups, called stacks, to obtain a usable voltage and power output. Unlike batteries, however, fuel cells do not release energy stored in the cell, running down when battery energy is gone. Instead, they convert the energy typically in a hydrogen-rich fuel directly into electricity and operate as long as they are supplied with fuel and oxidant. Fuel cells emit almost none of the sulfur and nitrogen compounds released by conventional combustion of gasoline or diesel fuel, and can utilize a wide variety of fuels: natural gas, coal-derived gas, landfill gas, biogas, alcohols, gasoline, or diesel fuel oil.
In transportation applications, SOFC power generation systems are expected to provide a higher level of efficiency than conventional power generators which employ heat engines such as gas turbines and diesel engines that are subject to Carnot cycle efficiency limits. Therefore, use of SOFC systems as power generators in vehicle applications is expected to contribute to efficient utilization of resources and to a relative decrease in the level of CO2 emissions and an extremely low level of NOx emissions. However, SOFC systems suitable for use in transportation applications require a very compact size as well as efficient thermal management. Thermal management must be accomplished whereby the outer surface of the fuel cell envelope is typically maintained below 45xc2x0 C. while the temperature inside the stack is about 700xc2x0 C. to about 950xc2x0 C.
As with fuel cells generally, very hot solid oxide fuel cells (SOFC) having high electrical conductivity, are used to convert chemical potential energy in reactant gases into electrical energy. In the SOFC, two porous electrodes (anode and cathode) are bonded to an oxide ceramic electrolyte (typically, yttria stabilized zirconia, ZrO2xe2x80x94Y2O3) disposed between them to form a selectively ionic permeable barrier. Molecular reactants cannot pass through the barrier, but oxygen ions (O2xe2x88x92) diffuse through the solid oxide lattice. The electrodes are typically formed of electrically conductive metallic or semiconducting ceramic powders, plates or sheets that are porous to fuel and oxygen molecules. Manifolds are employed to supply fuel (typically hydrogen, carbon monoxide, or simple hydrocarbon) to the anode and oxygen-containing gas to the cathode. The fuel at the anode catalyst/electrolyte interface forms cations that react with oxygen ions diffusing through the solid oxide electrolyte to the anode. The oxygen-containing gas (typically air) supplied to the cathode layer converts oxygen molecules into oxygen ions at the cathode/electrolyte interface. The oxygen ions formed at the cathode diffuse, combining with the cations to generate a usable electric current and a reaction product that must be removed from the cell (i.e., fuel cell waste stream). Typical reactions taking place at the anode (fuel electrode) triple points are:
H2+xc2xdO2xe2x88x92xe2x86x92H2O+2exe2x88x92
CO+xc2xdO2xe2x88x92xe2x86x92CO2+2exe2x88x92
The reaction occurring at the cathode (oxygen electrode) triple points is:
xe2x80x83xc2xdO2+2exe2x88x92xe2x86x92O2xe2x88x92
The overall system reactions in the cell are:
xc2xdO2+H2xe2x86x92H2O
xc2xdO2+COxe2x86x92CO2
The consumption of the fuel/oxidant ions produces electrical power where the electromotive force is defined by the Nernst equation:   E  =      Exc2x0    +                            R          ⁢                      xe2x80x83                    ⁢          T                          2          ⁢          F                    ⁢              xe2x80x83            ⁢      ln      ⁢              xe2x80x83            ⁢              (                              P                          H              2                                ⁢                                    P                              O                2                                            1                /                2                                      /                          P                                                H                  2                                ⁢                O                                                    )            
SOFC fuel stacks typically operate in the relatively high temperature range of about 700xc2x0 C. to about 950xc2x0 C. Reactant gases are preheated, typically by heat exchangers, to prevent the gases from cooling the stack below the optimum operating temperature. The heat exchangers, whether discrete or part of the total SOFC furnace, can be quite bulky, complex and expensive. In a traditional heat exchanger design, hot exhaust gas from the electrolyte plate is fed to the heat exchanger, and preheated reactant gas is received from the heat exchanger via costly insulated alloy piping. The heat exchanger and piping also require considerable installation and maintenance expense. High temperature piping and heat exchangers are costly from the standpoint of heat loss as well. The piping and heat exchanger have considerable surface area where heat may be exchanged with the atmosphere. This heat is thus unavailable to preheat incoming gases.
A similar situation exists for the fuel processing system in a SOFC. Partially reformed fuel gas (i.e., vaporized gasoline at a near stoichiometric fuel/air ratio) is typically reprocessed or reformed to convert some of the reaction products into usable hydrocarbon feedstock. Reformation of hydrocarbons often is endothermic and often requires temperatures in excess of about 750xc2x0 C. to achieve complete conversion of the hydrocarbon feed. Most SOFC systems must burn a portion of the fuel for the purpose of supplying the heat of reaction for the reformation process. The high temperature reformer unit and associated piping also have considerable surface area subject to heat loss. Such designs are very bulky, expensive and have limited ability for passive thermal feedback control.
Simple laminate SOFC stack enclosures of stainless steel and ceramic insulation fibers with internal and/or external heat exchangers for fuel and oxidant streams have been developed. For example, U.S. Pat. No. 5,340,664 to Hartvigsen discloses an insulative enclosure housing an SOFC system and heat exchangers used to capture and remove thermal energy from the fuel cell system. The heat exchangers include a screw culvert heat exchanger and a plate fin heat exchanger.
U.S. Pat. No. 5,366,819 to Hartvigsen et al. discloses a reformer located inside a fuel cell stack furnace for breaking down hydrocarbon feedstock into fuel for the fuel cells. Heat recuperated from the oxidation process in the fuel cell stack is used to support the endothermic reformation reaction in the reactor beds of the reformer. Heat transfers to incrementally shielded packed beds of the reformer by radiation from the stack and furnace wall and by forced convection from the exhausting airflow exiting the stack furnace. Temperature gradients in the reformer are controlled by incremented radiation shielding apparatus and by counterflow heat exchange. The counterflow heat exchanger forms the inner wall of the stack furnace and comprises a corrugated fin type heat exchanger captured between two layers or walls of high temperature sheet metal.
What is needed in the art is a compact, efficient SOFC thermal management and fuel reformation system. What is further needed in the art is a compact, efficient SOFC thermal management and fuel reformation system suitable for transportation applications. What is further needed in the art is a compact, thermally efficient fuel reformation system that can be adjusted in accordance with the SOFC stack design and reaction requirements of different fuel feedstock.
The present invention provides a compact integrated thermal management and fuel reformation system for SOFC systems and is particularly advantageous for SOFC systems used in transportation applications. In the present system, a SOFC stack comprising a plurality of solid oxide bicells is disposed within an interior chamber of an envelope structure formed by a thermally insulative wall.
A recuperator having dual function as a radiation-dominant heat exchanger and fuel reformer is disposed within the envelope in fluid and thermal communication with and close-coupled to the SOFC stack. The recuperator has interior walls that are color graded to effect a plurality of temperature zones forming a positive temperature gradient in the direction of said solid oxide fuel cell stack.
A mixed mode heat exchanger is disposed between the recuperator and the insulative wall in fluid communication with an oxidant source and the recuperator. Together, the insulative wall, the mixed mode heat exchanger, and the recuperator form an xe2x80x9cadiabatic wallxe2x80x9d such that the system functions substantially without loss or gain of heat to the dictates of the power loads.
During operation of the system, oxidant (typically air) enters the mixed mode heat exchanger through an oxidant inlet and is partially preheated therein. The partially preheated oxidant flows through a passage to the recuperator where it is further heated. Fuel gas is supplied to a selected recuperator zone and partially or fully reformed therein to provide a tailored anode fuel stream for the SOFC.
Manifolds comprising oxidant and fuel channels for passing preheated oxidant and reformed fuel from the recuperator to the SOFC oxidant and fuel inlets are arranged so as to effect increased fuel flow in peripheral portions of each fuel cell adjacent the recuperator. The channels are further arranged so as to effect increased oxidant flow toward the center of each fuel cell for cooling the SOFC stack.
The present system provides conversion of fuel (such as gasoline) to electricity at high efficiency using a compact envelope whose components have dual roles. The present system simplifies heat management by utilizing passive thermal feedback and ameliorates differential heat distribution within the fuel cell stack. Further, the present system provides a vastly superior and compact device as compared with discrete component SOFC systems or integrated screw culvert or fin-type heat exchangers disposed within an SOFC furnace.
The smaller volume of the present SOFC system is particularly useful for transportation applications. For example, the present system is particularly advantageous for use in power generation systems comprising hybrid electric powertrains such as those disclosed in commonly assigned U.S. Pat. No. 6,230,494 entitled xe2x80x9cPower Generation System and Methodxe2x80x9d and in commonly assigned, co-pending U.S. patent application Ser. No. 09/241,239 entitled xe2x80x9cPower Generation System and Method With Exhaust Side Solid Oxide Fuel Cellxe2x80x9d, and commonly assigned, co-pending U.S. patent application Ser. No. 09/294,679 entitled xe2x80x9cPower Generation System and Methodxe2x80x9d, all of which are hereby incorporated by reference herein in their entireties.
The present tailored anode fuel stream results in less stress on stack components from fast, large thermal excursions and balances thermal management across the face of each anode. Reformation is selectively endothermic or exothermic based upon the power demand to the SOFC stack, thus controlling heat flow into and out of the SOFC stack and inhibiting heat flow from within the system to the outside.
Further, the reformer may be employed as a pyrolysis unit to quickly bring the SOFC stack up to operational temperatures (about 700 to about 950xc2x0 C.) from start-up (about 550xc2x0 C.) by thermally catalyzing the cracking of gasoline, producing a nearly pure hydrogen fuel stream.
In a preferred embodiment, the present system and method provide the further advantage of eliminating vertical temperature gradient in the SOFC stack by utilizing segregated specialized anode fuel streams for selected SOFC stack sections. In a more preferred embodiment, segregated specialized anode fuel streams are employed in combination with reflective surfaces disposed upon selected interior portions of the chamber wall and radiant energy absorbing surfaces of the SOFC stack end sections. Such reflective and absorbent surfaces are arranged so as to reflect radiant energy emitted from warmer, center stack portions to cooler end stack sections. In combination, the present features effect thermal management via passive feedback control of photons. The system may be tailored for particular SOFC stack design and fuel requirements.
These and other features and advantages of the present invention will be apparent from the following brief description of the drawings, description of the preferred embodiment, and appended claims and drawings.